A fatty acid is a carboxylic acid with a long chain of unbranched aliphatic tail which can be saturated or unsaturated. Fatty acids have hydrocarbons in the aliphatic chain and it consist of elements carbon (C), hydrogen (H) and oxygen (O) which are arranged as a carbon chain skeleton with a carboxyl group (-COOH) at one end hence the head and tail. The lengths of these chains may vary depending on the number of carbons that they contain and they can range from a ten carbon chain to a thirty carbon chain but the most common ones are those with carbons between twelve and eighteen.
These fatty acids have both the polar and non-polar ends. The non-polar hydrocarbon chain is alkaline and the head is acidic but the hydrocarbon gives the molecule an overall non-polar character. Fatty acids bond with a glycerol molecule in order to form a fat usually called a triglyceride if all the three carbon atoms are attached to a fatty acid. But if the fatty acids are not in combination with any substance, they are then referred to as free fatty acids.
Fatty acids can be saturated or unsaturated, this is because of the double bonds which some of them contain in the hydrocarbon chain. Therefore unsaturated fatty acids have either one, two or more double bonds in their chain unlike the saturated fatty acids which do not contain any double bond in their structure. The position of the bonds is also a determining factor especially in melting properties of fatty acids. This therefore will influence other physical properties of fatty acids for example the melting point.
Melting points: Because of the nature of bonds that fatty acids have, unsaturated fatty acids have a lower melting point as compared to saturated fatty acids. The melting points of saturated fatty acids follow the boiling point principle. This also depends on the molecular geometry when they bond, for example the tetrahedral bonds that appear linear but have some zigzags in saturated fatty acids. This kind of orientation therefore leads to higher melting point in saturated fatty acids as compared to unsaturated. Saturated fatty acids allow for close stacking of their molecules thus close intermolecular interactions which means that they will have a higher melting point.
For unsaturated fatty acids, the presence of double bonds makes their intermolecular structure to be bent, thus the geometry of the double bond follows a cis configuration which is weak. They do not allow for close stacking which therefore means that they need a less amount of energy for them to melt as compared to saturated fatty acids.
There are two configurational isomers of fatty acids which are the Cis and Trans isomers. These terms are used to describe the orientation of the hydrogen atoms with respect to the double bond. Cis means the hydrogen atoms will lay on the same side whereas trans means the hydrogen atoms lay across or on either side. Naturally occurring fatty acids have the Cis isomer and it is good for health as compared to the trans isomer.
a. Short chain fatty acids - these are fatty acids whose hydrocarbon chain consist of less than six carbons for example butyric acid which is found in butter fat (cow milk).
b. Medium chain fatty acid - they contain between six and twelve carbon atom in their hydrocarbon chain.
c. Long chain fatty acids - These are fatty acids that contain more than twelve carbon atoms in the hydrocarbon chain but they may be less than twenty two carbon atoms.
d. Very long chain fatty acids - are those that contain more than twenty two carbon atom in the hydrocarbon chain. From these it should be noted that the length of the fatty acid chain will determine the melting point of the fatty acid, hence the longer the chain the higher the melting point and vice versa.
Fatty acids can further be classified into essential and nonessential fatty acids. Essential fatty acids are those that humans can not synthesize but can only be supplied through the diet. They include omega three fatty acids and omega six fatty acids. But omega nine is non essential since the body can manufacture enough amounts.
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